Glass photolithographic masks or plates comprising a chrome pattern on a glass or quartz substrate are used in the manufacture of thousands of semi-conductor wafers during a production run using a "stepper" printing machine. Therefore, it is critical that the surface of the mask be free of contaminating particles lest the image of the particles be reproduced on each wafer. Accordingly, the masks are typically inspected before a production run using very precise equipment shown, for example, in U.S. Pat. Nos. 4,943,734; 4,794,264; 4,794,265; and 5,389,794 incorporated herein by reference.
As a mechanical mask holder/spindle assembly spins the mask, the surface of the mask is scanned by a laser beam directed to the surface and the scattering of the laser beam off the surface is analyzed: the scattering off the surface will be different if a flaw or particle is present than if no flaw or particle is present. The scattering can be analyzed to the point where particles or flaw are classified by size. In order to scan for and detect very small particles (e.g. 0.3 microns in diameter), it is very important that the laser beam be focused to form a very small spot size.
Prior surface inspection apparatus utilized various signal processing routines and computer programs to process and analyze the particle data (e.g., size and position) thereby providing the user with a computer generated map which shows the relative size and position of all particles on the mask. The user then uses a microscope to visually inspect any particles of interest. These prior systems, however, were fairly slow. If the photolithographic mask is scanned slowly (e.g. much less than 1800 rpm) and the spot size of a laser beam is quite large (e.g. much larger than 1.7.times.5.3 micrometers) then the particle size and position data could be processed and analyzed during each scan. These low rates of scanning, however, mean that the inspection of each mask is slow and hence expensive. And, such a large laser beam spot size may fail to accurately detect very small particles.
A faster speed of rotation and a small laser beam spot size, however, makes it impractical to process and analyze the particle data during each scan.